1. Field of the Invention
The invention relates to a process and apparatus for producing hollow glass microspheres, and more particularly to techniques in which fine solid particles of a suitable glass are expanded by a heat treatment.
2. Description of Related Art
A longstanding difficulty inherent in these techniques resides in obtaining a product exhibiting well-defined, very homogeneous properties, by a minimum of simple operations. In particular, although processes involving formation of microspheres by passing the particles through a burner has been attempted and has appeared to have certain advantages, this type of operation has not proved to be very satisfactory with respect to its yield and the quality of the products obtained. This is explained particularly by the difficulty of performing the treatment in a perfectly uniform way for all the particles. A considerable part of the mass of treated particles is not found to be in expanded form on coming from the treatment.
Quality products can be obtained under previously known conditions, but for this purpose it is ordinarily necessary to add to the heat expansion treatment very rigorous sorting operations which limit the yield and substantially increase production cost.
To improve the yield of the particle expansion process, a solution previously considered consists in extending the heat treatment time. But it is found that even if a larger proportion of the treated particles is expanded, this way of operating entails increased risks of the particles, which are thus kept in a viscous state, sticking to the walls of the apparatus in which they are produced, or even sticking to one another to form undesirable aggregates.
Another difficulty comes from the fact that the expanded particles, when held at a high temperature too long, can allow the expansion gas to diffuse, which leads to deflation, and thus to denser particles.
To minimize these difficulties, previously proposed solutions have tended to limit the treatment temperature, which consequently has imposed more rigorous limits in regard to the initial composition and the size of the particles that can be treated effectively.